gen_init_cpio: attempt copy_file_range for file data

The copy_file_range syscall can improve copy performance by cloning
extents between cpio archive source and destination files.
Existing read / write based copy logic is retained for fallback in case
the copy_file_range syscall is unsupported or unavailable due to
cross-filesystem EXDEV, etc.

Clone or reflink, as opposed to copy, of source file extents into the
output cpio archive may (e.g. on Btrfs and XFS) require alignment of the
output to the filesystem block size. This could be achieved by inserting
padding entries into the cpio archive manifest.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://lore.kernel.org/r/20250819032607.28727-4-ddiss@suse.de
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

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